Capacitive micro electrical mechanical system (MEMS) switches possess potential to compete with conventional solid-state switching devices, such as positive-intrinsic-negative (PIN) semiconductor diodes and various field effect transistors (FET). Advantages of using MEMS are both of performance and cost, potentially offering high transmission current with low transmission line loss and distortion at low fabrication and implementation costs.
Capacitive MEMS switches are micro devices whose active element is a thin conductive membrane suspended above a substrate, movable upon electrostatic force generated through the application of an electrostatic field between the substrate and the membrane. Microscopic switching of electrical signals between transmission and termination is thus achieved by physically connecting or disconnecting one or two electrical leads through contact of the deformed membrane, or its metallic tips, with the electrical leads. As the membrane is set to operate and provide means for physical switching between connection and disconnection via mechanical deformation under electrostatic force, electrical interference and thus induced cross-talk noise upon switching and transmitting electrical signal must be minimized in such microdevices (which many of the prior art fail to address effectively, for example, the disclosure by U.S. Pat. No. 6,452,124)
Micro structural behavior of the deformable membrane relative to the substrate in electrical-mechanical aspects subject to electrostatic forces and external disturbance is the first key to achieving effective and reliable performance of capacitive MEMS switches. Mechanical stiffness, toughness against fatigue and shock, and their temperature dependence and even residual stress of the deformable membrane are among many critical factors to encounter and control. The effective contact of the deformed membrane with the electrical leads is also critical to the performance of capacitive MEMS switches when electrostatically actuated. Many other factors limit such physical contact and thus its electrical characteristics including contact resistance and variation as well as thus induced signal noise. Some key aspects relate to certain material and interfacial properties of contacting conductive surfaces; surface roughening and hillocking as well as surface degradation such as oxidation are absolutely negative factors to the quality and consistency of such critical contact. Those material and surface properties become of importance when forming conductive thin films, in particular, the electrical leads and the deformable membrane as well as the contact tips using metallic thin films.
All of those factors pose stringent requirements for material selection, structural design and fabrication process of capacitive MEMS switches. Meanwhile there is also a need for designing and fabricating capacitive MEMS switches based on single crystal silicon wafer manufacturing process platforms and in particular, produced and integrated with CMOS microdevices on one silicon wafer while CMOS microdevices provide electrical control and driving to capacitive MEMS switches.
In the prior art and industrial practice to date, most of the available design and fabrication methodologies are based on bulk MEMS process platforms, which separate fabrication of MEMS from CMOS wafer process, resulting in that the capacitive MEMS switches cannot be produced and integrated with CMOS microdevices on one silicon wafer.